In internal combustion engines with autoignition or compression-dependent ignition, the start of the combustion depends on various operating parameters such as, for example, cylinder pressure, temperature, oxygen concentration and, in engines with direct injection, timing of fuel injection. Many effects of the combustion process, for example, the torque, the emissions and combustion noise, depend on the start of combustion. For this reason, it is important to detect and regulate the start of combustion.
In the motor vehicle engines that are currently on the market, the start of combustion is not measured and is controlled only indirectly by adapting the timing of the fuel injection on the basis of the engine speed and torque.
Such open loop control is not sufficiently robust as, for example, reacting to changes in the fuel quality or imprecisions in the fuel injection devices which lead to deviation in the start of injection.
Known laboratory methods for determining the start of combustion are generally based on calculations of the heat release rate based on cylinder pressure signals, the start of combustion being defined as that point at which the heat release rate changes from negative to positive. Alternatively, the heat release rate can be extrapolated such that, the intersection between this gradient and the zero line defines the start of combustion. Furthermore, the start of combustion can be determined by differentiation of the cylinder pressure profile. The detection is easier and more precise if the difference between the cylinder pressure and an isentropic compression (without combustion) is used.
In addition, EP 1,132,605 A2 discloses a method for determining the start of combustion in which a continuous comparison takes place between a current measured value and a continuous averaged value of the previous measured values. If in the process a difference above a predefined threshold value occurs, that measured point among the previous measured points which is the first above the average value is searched for and it is assumed that the combustion has started at the corresponding time. A disadvantage here is that this method is susceptible to faults with respect to random fluctuation of the cylinder pressure signal.